The blue ice cave in Iceland: where it is and what you actually see

Snow is white because it is mostly air. Each flake is a little structure with gaps in it, and light bouncing around inside those gaps comes back out in every wavelength at once, which the eye reads as white.

Now bury that snow. More falls on top of it, and more on top of that, year after year. The layers below are compressed, the gaps close, and the air is gradually squeezed out. After enough decades what is left is glacial ice: dense, almost air-free, and behaving quite differently towards light. It absorbs the long wavelengths — red, orange, yellow — and scatters back the short ones. What comes out is blue.

This is why the guides take groups to particular caves and not others. The strongest colour is in the oldest, deepest, densest ice, and a cave cut into that ice glows in a way that a cave near the surface simply does not. It is also why the colour changes as you walk in: the further from daylight, the more the ice has to work on the light, and the deeper the blue goes.

One honest note about photographs. A phone on automatic will usually make an ice cave bluer than it looked, and a photographer with a light aimed at one wall can make it look bluer still. The colour is real. The saturation in the image that brought you here probably is not.

A blue ice cave is not a cave in rock that happens to contain ice. It is a hole inside the glacier itself. In summer, meltwater runs over the surface, finds a crack, and disappears into it; underneath, it carves a channel through the ice on its way to the sea. In winter the melt stops, the channel drains, and what is left is a passage with walls of blue ice and a floor of the black sand the water was carrying.

Nearly all of Iceland’s famous caves sit in Vatnajökull, the largest ice cap in Europe, and specifically in its outlet glaciers on the southern side. Breiðamerkurjökull — the glacier that calves into the Jökulsárlón lagoon — produces most of them. Falljökull and Svínafellsjökull, reached from Skaftafell, produce others.

That geography has one practical consequence that catches people out constantly: the caves are on the south-east coast, not near Reykjavík. The lagoon is 380 kilometres from the capital. Skaftafell is an hour closer. Neither is a comfortable day trip from the city in winter, which is why the eleven-hour coach tours and the multi-day trips on this site exist at all.

There is one exception worth knowing about. The Katla cave, in Mýrdalsjökull above Vík, is a different kind of cave in a different kind of ice, much closer to Reykjavík, and it can be visited all year. The ice there is darker and more heavily ash-banded than the crystal caves, and people who booked it expecting the photograph on this page are sometimes surprised.

The black. Iceland is volcanic, and ash from past eruptions lands on the glaciers and is buried by the next winter’s snow. A cave cut down through those layers has black bands running through the blue, and a floor of black sand washed down by the same meltwater that made the passage. Most people find it more interesting than the pure blue they expected, but it is a surprise if nobody has mentioned it. It is also what stops the floor from being a skating rink.

The time. A three-hour ice cave tour is roughly forty minutes of driving each way, fifteen to twenty-five minutes of walking each way, and twenty to forty minutes under the ice. That is the shape of it, on every operator. People who book knowing this are almost never disappointed; people who imagined a half-day underground write the reviews that say the tour was too short. If you want more time inside, the five-and-a-half-hour tour at $259 is the one that gives it to you, and when conditions allow it visits more than one cave.